The present invention relates to a decoding method or a code error correcting method in an apparatus in which digital signals are recorded and reproduced, and in particular, to a code error correcting method suitable for improving the decoding efficiency in the double-encoding code system.
In an apparatus for recording and reproducing digital signals, error correction codes are commonly used to increase the system reliability. Such codes have been applied to, for example, a digital audio disk (CD) and a digital VTR in which audio and video signals are digitized for the recording and, reproducing operations. In such an apparatus, due to causes such as a dropout, a slight random error and a consective code error (to be referred to as a burst error herebelow) take place. To an apparatus in which these types of code errors may occur at the same time, the double-encoding code system including error correction codes arranged in a two-dimensional configuration is applied in many cases. In the double-encoding code system, the error correction can be achieved in two stages when decoding the codes, which realizes a code configuration having a very high efficiency with respect to a complex error.
Prior-art techniques for correcting code errors have been described, for examples, in the JP-A-No. 57-24143 and JP-A-No. 57-10557.
FIG. 4 is a schematic diagram showing an example of an arrangement including data 11 and first parity symbols 12A and second parity symbols 12B in the double-encoding code system. The first parity symbols 12A comprise two parity symbols A10 and A20 for a group of ten symbols vertically arranged as 00, 10, 20, ---, 80, and 90, where each symbol is constituted from eight bits. The second parity symbols 12B include two parity symbols B00 and B01 for a group of ten horizontally arranged symbols 00, 01, 02, ---, and 09. As the code, the Reed-Solomon code defined on a Galois field GF (2.sup.8) may be used, for example. The Galois field is a field having a finite number of symbols. For example, GF (2.sup.8) indicates that the field has symbols of which number is 2.sup.8 -1=255. Further each symbol consists of eight bits. When this code is applied to the parity in the code configuration of FIG. 4, there is provided a function to correct an error of at most a symbol in the vertical or horizontal direction.
When recording and reproducing (or transmitting) the data in such an arrangement, data of the first row 00, 01, ---, 09, B00, B01 constituting a code word is first recorded for reproduction, and the subsequent data are similarly recorded as a code word of the second row, a code word of the third row, ---, and a code word of the 12th row in this order. That is, a code word here indicates the symbols constituting a row or a column of FIG. 4.
A random error occurs in an arbitrary symbol, whereas a burst error consecutively takes place in the horizontal direction (along the row). FIG. 5 shows an example of such a burst error. In this case, the function to correct errors can be further improved by specifying the row in which the error has occurred. For example, in the case of FIG. 5, if the number of rows in which errors have occurred (the number of erasure flags) is at most two, all errors can be corrected by use of the first parity 2A (in the vertical direction). The number is limited to 2 because the number of parity symbols Alj and A2j of the vertical parity 2A is two. This operation is called an erasure correction. The erasure correction has a very high capability to correct burst errors; however, when the number of random errors increases, there arises a problem that the error correcting capability is rapidly lowered. FIG. 6 is a schematic diagram illustrating an example including a greater number of random errors. In this case, since the number of rows in which errors have occurred, namely, the number of erasure flags is three or more, the erasure correction is impossible.
On the other hand, in a random error correction, only the vertical parity may be used to correct the 1-symbol error. Namely, in a case where only the random errors of FIG. 6 have occurred, the 1-symbol error takes place in the vertical direction, and hence all errors can be corrected. However, for such a long burst error as shown in FIG. 5, a 2-symbol error occurs in the vertical direction and thus the error correction is impossible.
As described above, although there can be considered several error correcting systems for decoding the codes of the double-encoding code system, these correcting systems are respectively attended with advantages and disadvantages.
When the number of code errors increases due to deterioration of the condition of the apparatus and medium, there may result error problems that cannot be detected or the correct data is mistakenly subjected to an error correction (mis-correction). To overcome these problems, a method is applied to such apparatus in which code errors beyond the correcting capability of the apparatus are detected so as to be replaced (concealed) with signals having a strong correlation (for example, image data contained in the preceeding field), which can reduce the probability of the mis-correction and the error detection failure. However, when the number of symbols to be replaced is increased, there exist problems such that the signal deterioration occurs in a portion where the screen changes (having no correlation).